Semiconductor pressure sensor and method of manufacturing semiconductor pressure sensor

ABSTRACT

A semiconductor pressure sensor includes a first substrate having a concave portion and an alignment mark at a main surface thereof, and a second substrate formed on the main surface of the first substrate and having a diaphragm provided to cover a space inside the concave portion of the first substrate and a gauge resistor provided on the diaphragm. The alignment mark is provided to be exposed from the second substrate. Accordingly, it is possible to obtain a semiconductor pressure sensor and a method of manufacturing the same with reduced production costs and with improved pressure measuring accuracy.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor pressure sensor and amethod of manufacturing a semiconductor pressure sensor, and moreparticularly to a semiconductor pressure sensor having a gauge resistorand a method of manufacturing a semiconductor pressure sensor.

2. Description of the Background Art

Conventionally proposed is a semiconductor pressure sensor including adiaphragm and a reference pressure chamber formed by bonding a flatplate-like silicon substrate and a silicon substrate having a concaveportion.

For example, Japanese Patent Laying-Open No. 2000-124466 proposes asemiconductor pressure sensor including a diaphragm and a referencepressure chamber. In the semiconductor pressure sensor described in thispublication, a first silicon substrate in the shape of a flat plate anda second silicon substrate having a concave portion are bonded with anoxide film formed on a surface of the first silicon substrate beinginterposed therebetween. The concave portion is sealed by the firstsilicon substrate to form a reference pressure chamber. Then, the secondsilicon substrate is ground to form a diaphragm at a portion that coversthe concave portion of the second silicon substrate. Gauge resistors areformed at prescribed positions of the diaphragm.

In one method of manufacturing a semiconductor pressure sensor asdescribed in the publication above, an alignment mark (first alignmentmark) is formed simultaneously with the concave portion at a firstsurface of the second silicon substrate that is to be bonded to thefirst silicon substrate. Registration is performed with respect to thefirst alignment mark using an IR (infrared) aligner, so that analignment mark concave portion (second alignment mark) is formed at asecond surface opposite to the first surface of the second siliconsubstrate. Registration is performed with respect to the secondalignment mark using a stepper, so that gauge resistors are formed atprescribed positions of the diaphragm.

In another manufacturing method described in the publication above, afirst alignment mark is formed to pass through the second siliconsubstrate. Registration is performed with respect to the first alignmentmark using a stepper, so that a second alignment mark is formed at thesecond surface of the second silicon substrate. Gauge resistors arethereafter formed in the same way as the one manufacturing method above.

In one method of manufacturing a semiconductor pressure sensor describedin the publication above, the concave portion cannot be visuallyidentified because the concave portion is shielded by the first andsecond silicon substrates. Therefore, the accurate position of thediaphragm covering the concave portion cannot be visually identified,either. Then, gauge resistors are formed at prescribed positions of thediaphragm using an alignment mark. An IR aligner is used to recognizethe first alignment mark since the first alignment mark cannot bevisually identified. A stepper having an IR alignment function is usedas the IR aligner. However, steppers having the IR alignment functionare extremely scarce. Even in the environment in which a stepper havingthe IR alignment function is available, the frequency of use of the IRalignment function is generally very low. Therefore, capital investmentefficiency is decreased and the unit cost per process is increased.Accordingly, the production cost is increased.

In order to match the thickness of the diaphragm with the desiredthickness, it is necessary to measure the thickness of the diaphragm orthe second silicon substrate, for example, by a light interferencemeasuring method while grinding the second silicon substrate. However,the amount of grinding the second silicon substrate varies. In addition,many concave portions, which are formed in the second substrate, vary indepth. Therefore, the thickness of the diaphragm varies significantly.Therefore, another problem is that it is difficult to finish thethickness of the diaphragm as desired with a good yield. Accordingly,the pressure measuring accuracy is reduced.

In another method of manufacturing a semiconductor pressure sensor asdescribed in the publication above, the first alignment mark is formedto pass through the second silicon substrate including the concaveportion and the diaphragm. The second silicon substrate has a thicknessof at least 10 μm or more. Therefore, in order to form the firstalignment mark, the second silicon substrate must be etched at least 10μm or more from the first surface which is the bonding interface. Thisrequires time and cost. Accordingly, the production cost is increased.

Furthermore, it is difficult to ensure the pattern accuracy of thealignment mark at the second surface of the second silicon substratesince the first alignment mark must be etched deeply to pass through thesilicon substrate from the first surface of the second siliconsubstrate. Therefore, there is another problem of deterioration of thealignment accuracy. Accordingly, the pressure measuring accuracy isreduced.

In both of one and another methods of manufacturing a semiconductordevice described in the publication above, gauge resistors are formedwith reference to the second alignment mark registered based on thefirst alignment mark. Therefore, there is a problem that theregistration accuracy of the gauge resistors is reduced as compared withwhen the gauge resistors are formed based on the first alignment markwithout the second alignment mark. Accordingly, the pressure measuringaccuracy is reduced.

Now, the method of etching a silicon substrate to form a concaveportion, etc. mainly includes dry etching and wet etching. With eithermethod, a minute amount of metal elements may contaminate a siliconsubstrate during etching or during cleaning after etching. In asemiconductor pressure sensor, a gauge resistor is formed by introducingan impurity in a silicon substrate by ion implantation, etc. The metalcontamination may lead to variations in characteristics of the gaugeresistor and reduction in reliability.

Supposing that the metal elements are unintentionally introduced intothe silicon substrate in the step of forming the concave portion, themetal elements move in the silicon substrate when the silicon substrateis processed at high temperatures of 1000° C. or higher in thesubsequent thermal processing step. Therefore, with such a configurationas the semiconductor pressure sensor described in the publication abovein which a concave portion is formed in the second silicon substrate onwhich gauge resistors are to be formed, the movement of metal elementsdescribed above occurs in the thermal processing step carried out afterbonding between the second silicon substrate and the first siliconsubstrate, so that the metal elements intrude into the inside of thegauge resistors. As a result, the characteristics of the gauge resistorsvary, or the reliability is reduced. Accordingly, the pressure measuringaccuracy is reduced.

SUMMARY OF THE INVENTION

The present invention is made in view of the problems as describedabove. An object of the present invention is to provide a semiconductorpressure sensor and a method of manufacturing the same with reducedproduction costs and with improved pressure measuring accuracy.

A semiconductor pressure sensor according to the present inventionincludes a first substrate having a concave portion and an alignmentmark at a main surface thereof, and a second substrate formed on themain surface of the first substrate and having a diaphragm provided tocover a space inside the concave portion of the first substrate and agauge resistor provided on the diaphragm. The alignment mark is providedto be exposed from the second substrate.

In the semiconductor pressure sensor of the present invention, the firstsubstrate has a concave portion and an alignment mark, and the secondsubstrate has a gauge resistor. In other words, the second substrate,which is different from the first substrate having the concave portionand the alignment mark, has the gauge resistor. Therefore, it ispossible to prevent metal contamination in the first substrate duringformation of the concave portion and the alignment mark from spreadingto the gauge resistor in the second substrate. Therefore, characteristicvariations of the gauge resistor and reduction in reliability can beprevented. Accordingly, the pressure measuring accuracy can be improved.

Furthermore, the diaphragm can be provided by grinding the secondsubstrate without being affected by variation in the depth of theconcave portion, because the first substrate has the concave portion andthe second substrate has the diaphragm. Thus, the accuracy in thicknessof the diaphragm can be improved. Accordingly, the pressure measuringaccuracy can be improved.

In addition, the alignment mark can be visually identified because thealignment mark is exposed from the second substrate. This eliminates theneed of an IR aligner. Accordingly, the production cost can be reduced.

In addition, the pattern accuracy of alignment can be ensured, and thealignment accuracy is therefore not reduced, because the first substratehas the alignment mark at the main surface. Thus, the registrationaccuracy for the gauge resistor can be improved. Accordingly, thepressure measuring accuracy can be improved.

In addition, the cost and time required to form the alignment mark canbe reduced as compared with when the alignment mark is formed to passthrough the second substrate, because the first substrate has thealignment mark at the main surface. Accordingly, the production cost canbe reduced.

In addition, the registration accuracy for the gauge resistor can beimproved by using the alignment mark as a reference for registering thegauge resistor, because the first substrate has the alignment mark atthe main surface. Accordingly, the pressure measuring accuracy can beimproved.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a semiconductor pressuresensor in an embodiment of the present invention.

FIG. 2 is a schematic plan view of the semiconductor pressure sensor inwhich a first substrate is in a wafer state in an embodiment of thepresent invention.

FIG. 3 is an enlarged view of the semiconductor pressure sensor, showinga P1 portion in FIG. 2.

FIG. 4 is a schematic cross-sectional view of the semiconductor pressuresensor, showing a P2 portion in FIG. 2.

FIG. 5 to FIG. 12 are schematic cross-sectional views sequentiallyshowing the steps in a method of manufacturing a semiconductor pressuresensor in an embodiment of the present invention.

FIG. 13 is a schematic cross-sectional view of a semiconductor pressuresensor of a first comparative example in an embodiment of the presentinvention.

FIG. 14 is a schematic cross-sectional view of a semiconductor pressuresensor of a second comparative example in an embodiment of the presentinvention.

FIG. 15 is a schematic cross-sectional view of a semiconductor pressuresensor of a third comparative example in an embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, an embodiment of the present invention will bedescribed based on the figures.

First of all, a structure of a semiconductor pressure sensor in anembodiment of the present invention will be described.

Referring to FIG. 1, a semiconductor pressure sensor in an embodiment ofthe present invention mainly includes a first substrate 1, a secondsubstrate 2, an oxide film 7, and metal wiring 8. Second substrate 2 isjoined to first substrate 1. First substrate 1 and second substrate 2are, for example, silicon substrates.

First substrate 1 has a substrate body 11 and an oxide film 12. Oxidefilm 12 is formed surrounding substrate body 11. In other words, oxidefilm 12 is formed on a surface of first substrate 1. First substrate 1has a concave portion 3 and alignment marks 4 at a main surface 1 a.Concave portion 3 is arranged at the central portion of main surface 1a. Alignment marks 4 are provided to be exposed from second substrate 2.Alignment marks 4 are arranged at opposite ends of main surface 1 a. Itis noted that alignment mark 4 may be arranged at least one of theopposite ends of main surface 1 a.

Second substrate 2 is formed on main surface 1 a of first substrate 1.Second substrate 2 has a diaphragm 5. Diaphragm 5 is provided to cover aspace IS in concave portion 3 of first substrate 1. The portion thatcovers space IS in concave portion 3 of second substrate 2 formsdiaphragm 5. Diaphragm 5 and concave portion 3 constitute a referencepressure chamber. It is noted that a concave portion and an alignmentmark are not formed in second substrate 2.

Second substrate 2 has gauge resistors 6. Gauge resistors 6 are providedon diaphragm 5. Gauge resistors 6 are arranged at positions overlappingwith outer edges 3 a of concave portion 3 as viewed from the directionin which first substrate 1 and second substrate 2 overlap each other.Gauge resistors 6 are formed to be able to detect strain of secondsubstrate 2. The semiconductor pressure sensor is configured such thatpressure is detected by gauge resistors 6 detecting strain of secondsubstrate 2. For example, in a case where second substrate 2 is ann-type silicon substrate, a p-layer of a pn diode formed in the n-typesilicon substrate is used as gauge resistor 6.

Oxide film 7 is formed on second substrate 2. Gauge resistor contactholes are formed in oxide film 7 so as to partially expose gaugeresistors 6. Metal wiring 8 is formed on oxide film 7 to fill in thegauge resistor contact holes. Metal wiring 8 is formed in contact withgauge resistors 6. Metal wiring 8 is electrically connected with gaugeresistors 6. Metal wiring 8 is configured such that a resistance changeof gauge resistor 6 due to deformation of diaphragm 5 can be externallytaken out as an electrical signal.

The semiconductor pressure sensor having the first substrate in a waferstate will now be described.

Referring to FIG. 2 and FIG. 3, a number of semiconductor pressuresensors are formed in semiconductor substrate 1 in a wafer state. Asemiconductor pressure sensor shown in FIG. 3 is formed by cutting out asemiconductor pressure sensor shown by a P1 portion in FIG. 2. Thesemiconductor pressure sensor shown in FIG. 3 corresponds to thesemiconductor pressure sensor shown in FIG. 1. Alignment marks 4 areprovided at an outer peripheral portion 1 b of first substrate 1 in awafer state. For example, four alignment marks 4 are provided atpositions 90° away from each other. Referring to FIG. 4, a semiconductorpressure sensor shown by a P2 portion in FIG. 2 has one alignment mark 4provided at outer peripheral portion 1 b of first substrate 1.

Alignment marks 4 may be provided only at outer peripheral portion 1 bof first substrate 1 in a wafer state. Generally, in an SOI (Silicon OnInsulator) substrate manufactured by bonding two silicon substratestogether, the reliability of bonding is reduced at the outer peripheralportions of the silicon substrates. Therefore, the outer peripheralportion of the silicon substrate that is thinly ground is removed. Outerperipheral portion 1 b forms a terrace portion. The width of the terraceportion is approximately 1 mm or more and 5 mm or less.

In general, an area about 5 mm from the outer circumferential end of asilicon substrate is not used since effective electric circuits cannotbe formed there in a semiconductor wafer process. In other words, anarea about 5 mm from the outer circumferential end of a siliconsubstrate does not form an effective area in which effective chips arearranged. The number of effective chips can be maximized by formingalignment marks only at the terrace portion which is not an effectivearea. Therefore, the number of effective chips can be increased byproviding alignment marks 4 only at outer peripheral portion 1 b offirst substrate 1 in a wafer state as compared with when alignment marks4 are provided at regular intervals on the entire main surface 1 a offirst substrate 1.

A method of manufacturing a semiconductor pressure sensor in anembodiment of the present invention will now be described. In thefollowing, a single semiconductor pressure sensor will be described asan example, for convenience of explanation, although a number ofsemiconductor pressure sensors are simultaneously manufactured on awafer.

Referring to FIG. 5, first substrate 1 having concave portion 3 andalignment marks 4 formed at main surface 1 a is prepared. First,alignment marks 4 are formed at main surface 1 a of first substrate 1.Alignment marks 4 may be formed at outer peripheral portion 1 b (FIG. 1)of first substrate 1 in a wafer state. The width of outer peripheralportion 1 b may be 5 mm or less from the outer circumferential end ofouter peripheral portion 1 b of first substrate 1 in a wafer state.Alignment marks 4 are used for registration when gauge resistors 6, etc.are formed later. Alignment mark 4 has a shape determined by a stepperso as to allow use of a stepper. Alignment marks 4 are formed by etchingfirst substrate 1, preferably, using a plasma etching device, etc.Alignment marks 4 do not have to be etched deeply as long as they can bevisually identified with a stepper, and a depth of about 1 μm willsuffice.

Registration is performed, for example, by a stepper with reference toalignment marks 4, so that concave portion 3 is formed on main surface 1a. Concave portion 3 subsequently serves as a reference pressure chamberas a pressure sensor, and the depth thereof is thus desirably about 5 to100 μm and can be set relatively arbitrarily within this range. As thedepth of concave portion 3 is shallower, the processing load is reduced,but the capacity of the reference pressure chamber is reduced toincrease pressure variations due to minute leak, that is, outputvariations. The depth of concave portion 3 is chosen considering thistrade-off relation. To form this concave portion 3, a plasma etchingdevice may be used in the same way as alignment marks 4. However,considering that the depth is somewhat as deep as 5 μm or more, a deepRIE (Reactive Ion Etching) device may be used through a Bosch process.Alternatively, instead of dry etching using a plasma etching device anda deep RIE device, wet etching using alkaline etching liquid such asTMAH (tetramethylammonium hydroxide) may be used.

Concave portion 3 and alignment marks 4 may be formed simultaneously. Inthe case where concave portion 3 and alignment marks 4 are formedsimultaneously, concave portion 3 and alignment marks 4 can be subjectedto photolithography simultaneously using the same photo mask. Therefore,the alignment accuracy (positional accuracy) of concave portion 3 cansignificantly be increased.

Oxide film 12 is thereafter formed on a surface of first substrate 1.After concave portion 3 and alignment marks 4 are formed, firstsubstrate 1 is thermally oxidized to form oxide film 12 on the entirewafer surface. The thickness of oxide film 12 is suitably about 0.2 to 1μm.

Next, referring to FIG. 6 to FIG. 10, second substrate 2 is formed onmain surface 1 a of first substrate 1 so as to have diaphragm 5 coveringspace IS in concave portion 3 and to expose alignment marks 4. Referringto FIG. 6, first, second substrate 2 is bonded to main surface 1 a offirst substrate 1 to seal concave portion 3. First substrate 1 andsecond substrate 2 are laid on one another for vacuum evacuation and arethermally oxidized under the environment of about 1100° C. to be bondedto each other. Thus, the reference pressure chamber is hermeticallysealed.

If metal elements are unintentionally introduced into the firstsubstrate during formation of concave portion 3 and alignment marks 4,even if the metal elements move under a high temperature during joiningbetween first substrate 1 and second substrate 2, the metal elements aretrapped in oxide film 12 and stay in first substrate 1 and do notintrude into second substrate 2.

Referring to FIG. 7, outer peripheral portion 1 b of second substrate 2is ground, for example, by a grinder. The thickness of second substrate2 is not entirely ground but ground such that a small amount of thethickness is left. This prevents alignment marks 4 from disappearing dueto grinding of second substrate 2 and further grinding of even firstsubstrate 1.

Referring to FIG. 8, after outer peripheral portion 1 b of secondsubstrate 2 is ground, second substrate 2 left at outer peripheralportion 1 b is completely removed by etching. In this etching, alkalineetching liquid such as TMAH that only etches silicon and does not etchoxide films is preferably used. Thus, alignment marks 4 covered withoxide film 12 are not etched. As a result of this step, outer peripheralportion 1 b serving as the terrace portion is completed, and alignmentmarks 4 are exposed so as to be visually identified.

Referring to FIG. 9, second substrate 2 is ground to be thinned. Inaddition, in order to finish the surface in a mirror-smooth state,polishing is performed, for example, by CMP (Chemical MechanicalPolishing). Diaphragm 5 is thus completed.

Although the size and thickness of diaphragm 5 depends on pressure to bemeasured, the size can be set to 100 to 500 μm and the thickness can beset to 5 to 20 μm, if pressures of 1 to 5 atmospheres are to bemeasured.

In the foregoing description, diaphragm 5 is formed by grinding secondsubstrate 2 to the desired thickness after outer peripheral portion 1 bserving as the terrace portion is formed. However, outer peripheralportion 1 b serving as the terrace portion may be formed after diaphragm5 is formed by grinding second substrate 2 to the desired thickness.

Referring to FIG. 10, gauge resistors 6 are thereafter formed ondiaphragm 5 using the exposed alignment marks 4. Registration isperformed with respect to the exposed alignment marks 4 using a stepper,so that gauge resistors 6 are formed at prescribed positions ofdiaphragm 5. Oxide film 7 is then formed on second substrate 2.

Referring to FIG. 11, registration is thereafter performed with respectto the exposed alignment marks 4 using a stepper, so that gauge resistorcontact holes are formed at prescribed positions of oxide film 7. Then,metal wiring 8 electrically connected to gauge resistors 6 is formedusing the exposed alignment marks 4. Registration is performed withreference to the exposed alignment marks 4 using a stepper, so thatmetal wiring 8 is formed at prescribed positions to fill in the gaugeresistor contact holes.

Referring to FIG. 12, the back surface on the side opposite to mainsurface 1 a of first substrate 1 is ground to remove the unnecessaryoxide film. Thus, a pressure sensor wafer is completed. The unnecessaryoxide film on the side surface of first substrate 1 is additionallyremoved, whereby the semiconductor pressure sensor shown in FIG. 1 isproduced.

In an embodiment of the present invention, both concave portion 3 andgauge resistors 6 can be formed using a stepper with respect toalignment marks 4, so that gauge resistors 6 are formed with highaccuracy at prescribed positions of diaphragm 5 provided to coverconcave portion 3.

The operation and effects of the embodiment of the present inventionwill be described in comparison with comparative examples.

Referring to FIG. 13, in a semiconductor pressure sensor in a firstcomparative example in an embodiment of the present invention, firstalignment marks 4 a are formed at a first surface 2 a of secondsubstrate 2. First alignment marks 4 a cannot be visually identifiedfrom the second surface 2 b side since they are formed at first surface2 a of second substrate 2. Therefore, an IR aligner is used to recognizefirst alignment marks 4 a. Accordingly, the use of an IR alignerincreases the production cost.

Referring to FIG. 14, in a semiconductor pressure sensor in a secondcomparative example in an embodiment of the present invention, firstalignment marks 4 a are formed to pass through second substrate 2.Second substrate 2 has a thickness larger than the thickness of firstalignment mark 4 a provided at the surface of second substrate 2 as inthe first comparative example because concave portion 3 and diaphragm 5are formed in second substrate 2. Therefore, as compared with the casewhere first alignment marks 4 a are formed at the surface of secondsubstrate 2 as in the semiconductor pressure sensor of the firstcomparative example, longer time and higher cost are required to formfirst alignment marks 4 a in the semiconductor pressure sensor of thesecond comparative example, thereby increasing the production cost.

In addition, it is necessary to deeply etch second substrate 2 sincefirst alignment marks 4 a are formed to pass through second substrate 2from first surface 2 a to second surface 2 b. In this etching, throughholes are formed in second substrate 2 so as to be tapered from firstsurface 2 a toward second surface 2 b. First alignment marks 4 a areformed in the through holes. Therefore, the shape of first alignmentmark 4 a deteriorates at second surface 2 h as compared with at firstsurface 2 a. Therefore, the alignment accuracy is reduced. Accordingly,the pressure measuring accuracy is reduced.

Both in the first and second comparative examples, the thickness ofdiaphragm 5 considerably varies depending on variations in the amount ofgrinding second substrate 2 and variations in the depth of concaveportion 3. Therefore, it is difficult to finish the thickness ofdiaphragm 5 as desired with a good yield. Accordingly, the pressuremeasuring accuracy is reduced.

Furthermore, in both the first and second comparative examples, concaveportion 3 and gauge resistors 6 are formed in second substrate 2.Therefore, metal contamination during formation of concave portion 3 insecond substrate 2 causes variations in characteristics of gaugeresistors 6 and reduces reliability.

Referring to FIG. 15, in a semiconductor pressure sensor in a thirdcomparative example in an embodiment of the present invention, a firstalignment 4 a is formed on a surface 1 d of first substrate 1 that doesnot serve as a bonding interface. Concave portion 3 is formed at mainsurface 1 a with respect to this first alignment mark 4 a using adouble-sided aligner. Then, first substrate 1 and second substrate 2 arebonded together to seal concave portion 3. Diaphragm 5 is thereafterformed by grinding second substrate 2. Then, a second alignment mark 4 bis formed at second surface 2 h of second substrate 2 with respect tofirst alignment mark 4 a using a double-sided aligner. Gauge resistors 6are formed with reference to this second alignment mark 4 b.

In a method of manufacturing the semiconductor pressure sensor of thethird comparative example, an IR aligner in the first comparativeexample is unnecessary, and first alignment mark 4 a passing throughsecond substrate 2 in the second comparative example is alsounnecessary. However, a double-sided aligner having registrationaccuracy lower than a stepper is used twice. Therefore, the registrationaccuracy for gauge resistors 6 is only achieved to such a degree thatthe registration by the double-sided aligner having essentially lowregistration accuracy is repeated twice. Accordingly, the pressuremeasuring accuracy is reduced.

In all of the first to third comparative examples, gauge resistors 6 areformed with reference to second alignment mark 4 b registered based onfirst alignment mark 4 a. Therefore, the positional accuracy of gaugeresistors 6 is reduced as compared with the case where gauge resistors 6are formed based on first alignment mark 4 a without second alignmentmark 4 b.

In the semiconductor pressure sensor in an embodiment of the presentinvention, first substrate 1 has concave portion 3 and alignment mark 4,and second substrate 2 has gauge resistor 6. Therefore, second substrate2, which is different from first substrate 1 having concave portion 3and alignment mark 4, has gauge resistor 6. Therefore, it is possible toprevent metal contamination in first substrate 1 during formation ofconcave portion 3 and alignment mark 4 from spreading to gauge resistor6 in second substrate 2. Thus, characteristic variations of gaugeresistor 6 and reduction in reliability can be prevented. Accordingly,the pressure measuring accuracy can be improved.

Furthermore, diaphragm 5 can be provided by grinding second substrate 2without being affected by variations in the depth of concave portion 3because first substrate 1 has concave portion 3 and second substrate 2has diaphragm 5. Therefore, the accuracy in thickness of diaphragm 5 canbe improved. Accordingly, the pressure measuring accuracy can beimproved.

In addition, alignment mark 4 can be visually identified becausealignment mark 4 is exposed from second substrate 2. This eliminates theneed of an IR aligner. Accordingly, the production cost can be reduced.

In addition, the pattern accuracy of alignment mark 4 can be ensured,and the alignment accuracy is therefore not reduced, because firstsubstrate 1 has alignment mark 4 at main surface 1 a. Thus, theregistration accuracy for gauge resistor 6 can be improved. Accordingly,the pressure measuring accuracy can be improved.

In addition, the cost and time required to form alignment mark 4 can bereduced as compared with when alignment mark 4 is formed to pass throughsecond substrate 2, because first substrate 1 has alignment mark 4 atmain surface 1 a. Accordingly, the production cost can be reduced.

In addition, the registration accuracy for gauge resistor 6 can beimproved by using alignment mark 4 as the registration reference forgauge resistor 6, because first substrate 1 has alignment mark 4 at mainsurface 1 a. Accordingly, the pressure measuring accuracy can beimproved. In particular, the improvement of positional accuracy of gaugeresistor 6 with respect to diaphragm 5 can reduce variations of sensoroutput at a pressure of zero (that is, offset output).

In the semiconductor pressure sensor in an embodiment of the presentinvention, alignment mark 4 is provided at outer peripheral portion 1 bof first substrate 1 in a wafer state. Therefore, the provision ofalignment mark 4 at outer peripheral portion 1 b that is not aneffective area can increase the number of effective chips.

In the semiconductor pressure sensor in an embodiment of the presentinvention, gauge resistor 6 is arranged at a position overlapping withouter edge 3 a of concave portion 3 as viewed from the direction inwhich first substrate 1 and second substrate 2 overlap each other.Therefore, the arrangement of gauge resistor 6 at an area in whichstress is increased can increase a resistance change of gauge resistor6. Accordingly, the pressure measuring accuracy can be improved.

In the method of manufacturing a semiconductor pressure sensor in anembodiment of the present invention, it is possible to preventcharacteristics variations of gauge resistor 6 and reduction inreliability due to metal contamination during formation of concaveportion 3 and alignment mark 4, because concave portion 3 and alignmentmark 4 are formed in first substrate 1, and gauge resistor 6 is formedin second substrate 2. Accordingly, the pressure measuring accuracy canbe improved.

Furthermore, diaphragm 5 can be formed by grinding second substrate 2without being affected by variations in the depth of concave portion 3,because concave portion 3 is formed in first substrate 1 and diaphragm 5is formed in second substrate 2. Thus, the accuracy in thickness ofdiaphragm 5 can be improved. Accordingly, the pressure measuringaccuracy can be improved.

In addition, alignment mark 4 can be visually identified because secondsubstrate 2 is provided to expose alignment mark 4. This eliminates theneed of an IR aligner. Accordingly, the production cost can be reduced.

In addition, the pattern accuracy of alignment can be ensured, and thealignment accuracy is therefore not reduced, because alignment mark 4 isformed at main surface 1 a of first substrate 1. Thus, the registrationaccuracy for gauge resistor 6 can be improved. Accordingly, the pressuremeasuring accuracy can be improved.

In addition, the cost and time required to form alignment mark 4 can bereduced as compared with the case where alignment mark 4 is formed topass through second substrate 2, because alignment mark 4 is formed atmain surface 1 a of first substrate 1. Accordingly, the production costcan be reduced.

In addition, the registration accuracy for gauge resistor 6 can beimproved by using alignment mark 4 as a reference for registering gaugeresistors 6, because first substrate 1 has alignment mark 4 formed atmain surface 1 a. Accordingly, the pressure measuring accuracy can beimproved. In particular, the improvement of positional accuracy of gaugeresistor 6 with respect to diaphragm 5 can reduce variations of sensoroutput at a pressure of zero (that is, offset output).

The method of manufacturing a semiconductor pressure sensor in anembodiment of the present invention further includes the step of formingmetal wiring electrically connected to the gauge resistor using theexposed alignment mark. Accordingly, the registration accuracy for metalwiring can be improved. The improvement of registration accuracy formetal wiring can prevent a pressure measurement error due to apositional shift of the metal wiring. Accordingly, the pressuremeasuring accuracy can be improved.

In the method of manufacturing a semiconductor pressure sensor in anembodiment of the present invention, alignment mark 4 is formed at outerperipheral portion 1 b of first substrate 1 in a wafer state. Theprovision of alignment mark 4 at outer peripheral portion 1 b that isnot an effective area can increase the number of effective chips.

In the method of manufacturing a semiconductor pressure sensor in anembodiment of the present invention, the width of outer peripheralportion 1 b is 5 mm or less from outer circumferential end 1 c of firstsubstrate 1 in a wafer state. Thus, the recognition of alignment mark 4by a stepper is compatible with the maximization of effective chip area.The reason is as follows. As for the visual identification of alignmentmark 4 by a stepper, alignment mark 4 can be recognized better as it isarranged on the more inner circumferential side of first substrate 1. Onthe other hand, second substrate 2 lying on the alignment mark should beremoved in order to expose the alignment mark. Therefore, the effectivechip area is reduced as alignment mark 4 is arranged on the more innercircumferential side of first substrate 1. Moreover, the portion wheresecond substrate 2 is removed becomes a steep and big step. As a result,gauge resistors 6, oxide film 7, metal wiring 8, etc. to be formedsubsequently may be deteriorated in the coverage shape or stripped away.As a result of elaborate experiments, the present inventors have foundthat the recognition of alignment mark 4 by a stepper is compatible withthe maximization of effective chip area if alignment mark 4 is arrangedin the terrace portion and if the size of width of outer peripheralportion 1 b is in a range within 5 mm from the outer circumferential endof first substrate 1.

In the method of manufacturing a semiconductor pressure sensor in anembodiment of the present invention, alignment mark 4 is exposed byetching second substrate 2 after grinding second substrate 2.Accordingly, it is possible to prevent alignment mark 4 fromdisappearing due to grinding of second substrate 2 and further grindingof even first substrate 1. Furthermore, the process time can beshortened, for example, by cutting using a grinder using a grindingwheel in addition to etching requiring a long process time, therebyimproving the production efficiency.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the scopeof the present invention being interpreted by the terms of the appendedclaims.

What is claimed is:
 1. A semiconductor pressure sensor comprising: afirst substrate having a concave portion and an alignment mark at a mainsurface thereof; and a second substrate formed on said main surface ofsaid first substrate and having a diaphragm provided to cover a spaceinside said concave portion of said first substrate and a gauge resistorprovided on said diaphragm, wherein said alignment mark is provided tobe exposed from said second substrate.
 2. The semiconductor pressuresensor according to claim 1, wherein said first substrate is in a waferstate, and said alignment mark is provided at an outer peripheralportion (1 b) of said first substrate in said wafer state.
 3. Thesemiconductor pressure sensor according to claim 1, wherein said gaugeresistor is arranged at a position overlapping with an outer edge ofsaid concave portion as viewed from a direction in which said firstsubstrate and said second substrate overlap each other.
 4. A method ofmanufacturing a semiconductor pressure sensor comprising the steps of:preparing a first substrate having a concave portion and an alignmentmark formed at main surface thereof; forming a second substrate on saidmain surface of said first substrate so as to have a diaphragm coveringa space inside said concave portion and to expose said alignment mark;and forming a gauge resistor on said diaphragm using said exposed saidalignment mark.
 5. The method of manufacturing a semiconductor pressuresensor according to claim 4, further comprising the step of formingmetal wiring electrically connected to said gauge resistor using saidexposed said alignment mark.
 6. The method of manufacturing asemiconductor pressure sensor according to claim 4, wherein saidalignment mark is formed at an outer peripheral portion of said firstsubstrate in a wafer state.
 7. The method of manufacturing asemiconductor pressure sensor according to claim 6, wherein a width ofsaid outer peripheral portion is 5 mm or less from an outercircumferential end of said first substrate in said wafer state.
 8. Themethod of manufacturing a semiconductor pressure sensor according toclaim 4, wherein said step of forming a second substrate includes thesteps of: grinding said second substrate; and exposing said alignmentmark by etching said second substrate after grinding said secondsubstrate.